1. Field of the Invention
The present invention relates to a heat transfer sheet for transferring letters, symbols, designs, patterns or the like to a substance to which any of them is to be transferred (hereinafter referred to as "transfer substance") .
2. Description of the Prior Art
A heat transfer sheet is used to transfer letters, symbols or designs to a transfer substance for the purpose of display and/or decoration. The heat transfer sheet has a sheet-like substrate such as paper or a plastic film, a thermally transferable layer being arranged on the substrate and a releasing layer for intervening between the substrate and the thermally transferable layer. Alternately, the heat transfer sheet has a sublimable transfer layer being arranged on the substrate. When letters, symbols or designs are transferred on a transfer substance by using the heat transfer sheet, some methods are available. One of the examples has the steps of forming desired letters, symbols or designs on the releasing layer on the substrate by a printing method such as silk screen printing, gravure printing or offset printing, and transferring them to a transfer substance. Another example has the steps of applying the thermally transferable layer onto the whole surface of the substrate, cutting out desired letters, symbols or designs from the resulting assembly, and transferring the cut-out pattern to a transfer substance.
The heat transfer sheet with the thermally transferable layer on the whole surface of the substrate has the advantage that desired letters, symbols or designs can be formed in a desired amount at a desired time. A computer-controlled automatic cutting machine is used for cutting out letters, symbols or designs. Some methods are available for this purpose. One of the examples has the steps of forming notches extending from the thermally transferable layer toward the substrate of the heat transfer sheet, separating letters, symbols or designs individually from the heat transfer sheet, and rearranging them. Another example has the steps of making notches only in the thermally transferable layer, and removing the unnecessary portions of the thermally transferable layer. In the former method, it is difficult to rearrange the individually separated letters, symbols or designs. Thus, the latter method involving notches only in the thermally transferable layer is more advantageous.
The latter method, however, poses the following problem: In a heat transfer sheet having a thermally transferable layer on a substrate via a releasing layer, if the thermally transferable layer is thick, its unnecessary portions are easy to peel off; if the thermally transferable layer is thin, its unnecessary portions are difficult or impossible to peel off.
When letters, symbols, designs, etc. are to be transferred using a heat transfer sheet to a large-area transfer substance for the purpose of display or decoration, particularly in the form of an advertisement or a billboard, there is generally used a heat transfer machine called the Heat Vacuum Applicator (H.V.A.). The H.V.A. has a transfer table, a framed rubber sheet covering an upper surface of the transfer table, and a heating portion covering the rubber sheet. The space defined by the upper surface of the transfer table and the framed rubber sheet is deaerated by a vacuum pump to become a vacuum area. Materials necessary for transfer, such as a transfer substance and a heat transfer sheet, are placed between the transfer table and the framed rubber sheet prior to the deaeration step. Deaeration for forming the vacuum area may be performed from the transfer table side, and/or from the frame side of the framed rubber sheet. The heating device generally includes a row of incandescent lamps.
The heat transfer using the H.V.A. is advantageous because it can easily perform on materials with a large area, especially materials for advertisements or billboards. A method for heat transfer by the H.V.A. has the steps of placing a transfer substance on the upper surface of the transfer table, laying a heat transfer sheet on the transfer substance so as to face downwardly the thermally transferable layer, and superimposing on the heat transfer sheet a porous material, such as a woven fabric, of a size large enough to cover the whole of the transfer substance and the heat transfer sheet. Then, the framed rubber sheet is laid on the porous material, whereafter the vacuum pump is actuated to form the vacuum area. Within the vacuum area, air is removed from the interface between the transfer substance and the heat transfer sheet, whereby the heat transfer surface of the heat transfer sheet is brought into intimate contact with the surface of the transfer substance, and the contact surfaces are adapted to each other. After the contact surfaces are sufficiently adapted, heat is applied from above the rubber sheet by the heating device, with the vacuum pump being operated, thereby carrying out heat transfer.
As described above, the heat transfer by the H.V.A. requires a porous material, such as a woven fabric, as a third material in addition to the transfer substance and the heat transfer sheet. The porous material is indispensable to promote deaeration from the interface between the transfer substance and the heat transfer sheet within the vacuum area and to cause the contact surfaces of them to be completely contacted and adapted.
Placing the porous material every time a transfer procedure is performed makes operation complicated and decreases the efficiency of operation. The placement of the porous material also causes wrinkles during the deaeration of the vacuum area, thereby impairing transfer.